The present invention relates to an apparatus for the treatment and transportation of a material, consisting of piece goods or granules of solid material, in an at least partly fluidized bed, which apparatus comprises a perforated trough bottom for the material and means for creating an upwardly directed air or gas flow through said trough bottom and said material.
In thermal air treatment of foodstuffs on an industrial scale, continuously operating air treatment apparatuses are utilized, which often operate according to the fluidized bed principle. The term thermal air treatment is here taken to mean refrigeration or freezing by means of cold air or gas, heating, drying or moistening by means of hot air, gas or steam etc. The product which is to be air-treated is in this connection conveyed in a continuous process chain and enters the fluidized bed in a first position in order then to be conveyed in the fluidized bed to a second position. When the product reaches the second position, it is ready-treated and leaves the fluidized bed. The treatment air is caused in the apparatus to flow from underneath and upwards through a perforated trough bottom and through the material located above this trough bottom, fluidizing of the material being achieved. The heat transfer is very good in the fluidized state, for which reason quick and efficient treatment (freezing/heating) is obtained.
In a truly fluidized state transportation of the product from the first position to the second position takes place automatically, in that the material is poured in at the first position and permitted to flow over a side wall/edge (overflow) at the second position. There between the material behaves like a fluid liquid thanks to fluidization. Often, however, a truly fluidized bed cannot be achieved, e.g. when the material which is to be treated consists of delicate, heavy and/or irregular products. The reason for this is that the air flow through the bed cannot always be controlled so that the requirements for refrigeration/heat treatment, damage to the product and transportation requirements are met. The bed then remains only partly fluidized, semi-fluidized, in which case the product will rest wholly or partly on the perforated trough bottom.
A number of apparatuses are known, the aims of which inter alia are to improve transportation in conjunction with the above problems. In a known type of apparatus, the perforated plate including its side walls/edges, i.e. the trough, or only the perforated trough bottom, is agitated by symmetrical movements backwards and forwards in the conveying direction of the material. This aims to provide better spreading and separation of the material, transportation being made easier due to the fluidizing effect. In another similar type of apparatus, the trough is agitated instead in a vertical direction to achieve the same effect.
A further way of solving the problem has been to provide the apparatus with an endless net conveyor belt, the part of the conveyor belt which runs in the conveying direction being arranged above the perforated trough bottom, while the return part is arranged under the same. Alternatively, the conveyor belt can be formed (normally of plastic) so that it forms both the conveyor belt and the perforated trough bottom.
In some simple apparatuses, the perforated trough bottom is not used, but only a net conveyor belt.
A known apparatus of the type stated above is shown in U.S. Pat. No. 4,283,923, which apparatus comprises both a plate as a basis for a fluidized bed and a net conveyor belt arranged above the plate, which net conveyor belt is disposed to be agitated symmetrically backwards and forwards in the conveying direction.
In U.S. Pat. No. 4,628,838 an apparatus set is shown for combustion of a material in a fluidized bed, a filter plate which forms an air-permeated base for the fluidizing bed being disposed to vibrate so that a turbulently fluidizing motion occurs, transportation of the combustion material from the inlet end to the outlet end being facilitated.
U.S. Pat. No. 4,821,654 shows a furnace with a fluidized bed, which is stated to be used for thermomechanical regeneration of foundry sand. The fluidizing effect is stated to be amplified by vibration of the bed bottom, the vibrations also being said to facilitate transportation of the sand from the inlet end to the outlet end.
A further apparatus for freezing food in a fluidized bed is shown in U.S. Pat. No. 5,447,040, a conveyor belt being disposed to be driven at least periodically in the opposite direction to the conveying direction of the food. The aim of this is stated to be to counteract the formation of ice in clumps.
The above-named methods and apparatuses certainly function well in principle for their purposes, but the apparatuses are expensive to manufacture, require maintenance and are difficult to clean. The cleaning aspect is very important, especially in the food industry.
The object of the present invention is to present an apparatus for, preferably continuous, treatment and transportation of a material, consisting of piece goods or granules of solid material, in an at least partly fluidized bed, the aforementioned problems being lessened or overcome.
Thus an apparatus is presented according to claim 1 in which agitation of a trough bottom backwards and forwards in the conveying direction is utilized to actively convey a material which is at least partly carried by the trough bottom. In a partly fluidized bed in particular, i.e. where fluidizing on the basis of an upwardly directed air flow is not sufficient for the material to flow along like a fluid, agitation/movement on the part of the trough bottom acts to move the material actively in the conveying direction.
The apparatus according to the invention is relatively cheap to manufacture and maintain, as no conveyor belt or belt-driving mechanism is required. Cleaning becomes distinctly easier, since the nooks which are difficult to reach can be avoided, for example on belt tracks under the belt which are otherwise never reached without dismantling the equipment. As the difference in hygiene compared with apparatuses with conveyor belts is considerable, the apparatus according to the invention has qualifications to be approved for new applications.
The conveying principle of the apparatus according to the invention is based on the fact that the trough bottom is caused to move in the conveying direction with an acceleration which is not sufficient to overcome the static friction force between the material conveyed and the trough bottom. The trough bottom will then carry the material with it in the movement in the conveying direction. When the trough bottom has reached an end position for the movement, the direction changes, it being caused to accelerate sufficiently quickly in the direction opposite to the conveying direction for the static friction force between the material conveyed and the trough bottom to be overcome. The material is then set in motion relative to the trough bottom.
It should be understood that all the accelerations mentioned relate to absolute values, the direction of the accelerations being evident from the context.
According to one aspect of the invention, the trough bottom is disposed to move as far opposite to the conveying direction as in the conveying direction, the trough bottom not exhibiting any net movement in or opposite to the conveying direction.
According to another aspect of the invention, transportation of the material on the trough bottom takes place solely due to the movement of the trough bottom opposite to and in the conveying direction, and due to the fluidizing effect. It is characteristic of the invention that no further conveying power is required, e.g. in the form of net conveyor belts with net movement in the conveying direction, a flow of carrier gas in the conveying direction or the like. However, there is naturally nothing to prevent the invention from being combined with such methods.
According to an embodiment of the invention, a second, lower trough bottom is disposed to move in relation to the upper trough bottom and is formed so that the air speed through the double trough bottom is partly limited and released in a controlled manner. By limiting the air partly, an increased air flow is obtained in the areas where the air is not restricted, with a defined total flow of air through the product bed. This contributes to increased separation of the material due to the fact that the material is caused to move in a vertical direction and that it strikes the trough bottom when it sinks downwards due to the temporary restriction in the upwardly directed air flow.
The separation principle of the apparatus according to this embodiment of the invention is based on the fact that the lower trough bottom is formed with apertures, preferably transverse slits, over the entire trough width and with homogeneous material lying in between. The apertures then function as a passage for the air and the homogeneous material as a barrier. This second trough bottom is disposed to be agitated backwards and forwards along the conveying direction at a frequency of 20-180 cycles per minute. These cycles result in that the air flow on each cycle is reduced momentarily, the treated and conveyed material being caused to land or at least move vertically downwards at the same frequency as the cycles.